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Richard Krynen, Michael McBeath; Rainbow Stripes: Categorical Perception of Color. Journal of Vision 2020;20(11):1792. doi: https://doi.org/10.1167/jov.20.11.1792.
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Introduction: When a viewer sees a rainbow, a common experience is to perceive stripes of color despite the stimulus being a smooth gradient of pure wavelength electromagnetic frequencies. We investigated the number of stripes that were drawn by both normal vision and color blind individuals.
Method: 33 normal vision and 12 color blind participants viewed a geometrically controlled 6-inch long gradient of pure wavelength colors created by refracted sunlight through a prism onto a white piece of paper. Participants were instructed to mark boundaries where each distinct color ended and another began.
Results: We found the typical number of stripes indicated by normal vision participants is 6 (µ = 6.4, σ = 0.3), and by color blind participants is 4 6 (µ = 4.7, σ = 1.2). These numbers are significantly less than the 7 prototypical ROYGBIV categories of color traditionally specified by Newton and others. We also found that the longer wavelength colors of red and purple are more salient with wider perceived stripes.
Discussion: The effect of seeing a limited number or color stripes confirms color as a prototypical example of categorical perception that is consistent with our limited number of types of color receptor neurons. Given that the electromagnetic frequencies of light wavelengths in a rainbow change as a smooth continuum, the perception of color stripes could be framed as a categorical illusion. It may also reflect functional divisions used to separate meaningful real-world distinctions, possibly such as bluish liquids, greenish plants, and reddish earth tones.
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